I made this addon to speed up my level prototyping phase. It allows the user to quickly create new geo, all snapped to a world space grid.

This looks as if it would allow very simple, Radiant-style blocking out of on-grid geometry in Blender.

The finished layout could then be exported as OBJ to Radiant, or just exported as Quake / Quake 3 .map with some matching exporter.

Using Blender for level design has just become a lot easier. On-grid geometry is a lot easier to work with, be it for automatic clipping with q3map2, or for better lightmaps. needless to say, the biggest advantage would be the speed with which you can generate a layout using this.